Sub-level open stoping

This article provides detailed design and development information on the Sublevel Open Stoping Mining Method along with key information regarding project economics and safety .

Sublevel Open Stoping originated in the early 1900’s in iron ore mines in Michigan, USA. It was developed over the years from a method originally known as short hole bench and train mining.Generally a large scale open stoping method, it is often referred to as long hole or blast hole stoping.

Background

Description

This method is usually considered as an alternate to sublevel caving when a lower dilution method is needed and where the rock is fairly competent. The production for a mine using sublevel stoping can range from 15- 40 tons/employee-shift, while one stope can produce greater than 25000 tons per month.
In general this method is fairly safe as the mining tends to retreat away from previously mined or unsupported areas. Also local ground support can be applied in areas where workers are expected to go as safety measure, though remote equipment such as LHD’s can travel in unsupported areas.[2]

Orebody Requirements

Ore bodies that are ideal for sublevel stoping are very tall and have a medium to narrow width, but can have an extensive length. The ideal width of the orebody should be greater than 6 meters but less than 30 meters. Most of the time this type of orebody has a dip greater than the angle of repose of the ore, or steep enough so that the ore can fall efficiently into the open stope. Preferably the dip would be between 60 and 90 degrees, however anything above 45 degrees may be adequate.

Sublevel stoping requires a midrange ore grade in order to pay for its development. This mining method can be used at a wide range of depths. The ore bodies must be made of competent rock, and be surrounded by competent wall rock. Due to the nature of the drilling and blasting of the rock it is best if the orebody has a regular outline with well defined edges, as this lowers the amount of dilution. Since this mining method requires that the ore be blasted on multiple occasions, it is recommended that the ore have a high compressive strength and few planes of weakness such as joints, bedding planes or faults.[2]

Advantages

Dilution may occur but 100% of the ore is usually recovered from the stope.

Pillars that are left in place can be removed once adjacent stopes have been backfilled.

Large scale blasting lowers costs.

Stopes may be drilled before any blasting, to allow for larger and more efficient blasts.

The early development can be done in ore rather than in waste.

Easily to mechanize and can use large equipment.

High productivity and efficiency, up to 110 tons/employee-shift, depending on the orebody.

Repetitive techniques help facilitate training and safety.

Mid-range mining cost (relative cost 20%).

Little exposure to hazardous conditions ie: easily ventilated.

Low Dilution (20%).

Reasonable to high recovery (75%-90%).

Disadvantages

Early production is low due to the lack of available drawpoints near the slot however production increases as new drawpoints are reached.

Initial recovery is usually 35-50%.

Not a highly selective method.

Requires extensive early orebody developments with high capital expenditures.

Inflexible mining plan.

Drilling requires precision and should deviate less than 2% on any hole.

Orebodies which are narrower, less than 6 meters in width, often have higher costs because there will be lower production for each blast.

Support pillars may be left in place when a large ore body being mined.

Design

Dimensioning

There are numerous factors that affect the open stope length and width dimensioning in sublevel stoping which include the principal stress directions, competence of the hanging wall, optimum drill pattern, orebody geometry and the drilling drift layout. Proper design and dimensioning of each sublevel in this stoping method is critical to ensure crew safety and adequate equipment sizing: knowledge from operators with past experience is essential. At this stage of stope planning, help should be sought from people with experience in similar ore bodies, rock mechanics experts, and experienced mine design personnel. When dimensioning the interval between subsequent sublevels both the height of the ore and the pillar width must be taken into consideration, with interval lengths varying between 45-125m.[2][5]

Drawpoints

In order to ensure a good draw point system, some important considerations must be taken into account. First off, within the constraints of the stope dimensions, the drawpoints must be designed with adequate spacing in order to guarantee uniform drawdown and maximum recovery. Secondly proper floor and roadway design (including type of surface, reinforcing, grade for water runoff, and orientation with respect to the main haulage way) must be adhered to so optimum loader maneuverability and ground stability is achieved and front end loaders are capable of working in a straight configuration. Proper design is also essential to ensure that there is enough expansion space for the blasted ore.

Slot Raising

In conventional sublevel stoping where vertical rings or rows of holes are blasted it is necessary to have a slot to allow for rock expansion. It can be done as a slot raise driven by conventional raising methods, by raise boring, drop raising or crater blasting. The slot usually extends from the extraction level to the back of the stope. Typically long hole slashing is used to expand the slot to the full stope width, with an average slot width of 4-5m.

Undercutting

Carrying out proper undercutting is critical to the success of sublevel stope production due to its effects on loading efficiency and production blasting. To ensure stability, it should not be carried out more than a few rings in advance of blasting.
[5]

Development

Stope Sequence

1. Initial stope development consisting of driving raises to the levels above (left) and then proceeding to drive a slusher drift, finger raises, and sublevel drifts(right).[6]

Raises will be driven up either side of the orebody, or through the middle to allow for the creation of sublevels. Each sublevel will be a drift that extends the length of the orebody. If the orebody is extensive in width, it can have multiple drifts on the same level. This is where production drilling will take place. Each level can be accessed by a ramp or shaft in the footwall so that equipment can move between the levels.

Vertical raises can be driven between the levels to allow for access and ventilation. Access raises are also driven to divide the stopes into blocks. Next a haulage drift and an undercut must be driven in the foot wall and in the ore respectively where the draw points will be placed. This drift will allow for the drawpoints to be accessed where the ore will be removed. Along this level an orepass should be developed in a central location, which allows for easy access for all the mobile haulage equipment.

The ore must be undercut to allow for creation of the funnels or drawpoints. This under cut will be used for space for the ore to expand into once it has been blasted. The undercut must go the entire length of the drift that will be mined mine.

2. The stope is now completely developed and the opening raise is extended from hanging wall to foot wall. [6]

The drawpoints will be funnel like shapes to allow for the ore to flow down and avoid getting stuck in place. The level found above the drawpoints is usually the first to be mined. A slot will be driven either along one side of the stope, or in the middle to allow for the ore to swell when broken.

3. The mining of the stope progresses from the opening raise toward the manway (left) and the stope is progressively being mined, with the pillar left as support (right).[6]

The first round of drilling will be right beside the slot and will retreat back towards the level access or edge of the orebody. As the lowest level starts to retreat the level above it can start to blast their rounds. This repeats for each successive level above. When required a pillar can be left either as waste or as a support pillar. When the mining of a single stop has been completed, the stope may be filled with back fill as needed depending on the rocks characteristics.

If the orebody is of a large enough extent in any direction, the body can be divided into a couple of different stopes. This allows for simultaneous mining in different areas. Another way to mine multiple areas simultaneously would be to start from the middle of the orebody and work back towards an access point at either end of the level. Although this requires that there be access for equipment at both ends of the drift, it does allow for a greater production.
[6][2]

Drilling

For sublevel stopping most drill patterns involve keeping the holes on a vertical or near vertical plane on each sublevel. The length of the hole to be drilled depends on the extent of the ore and the sublevel spacing. Generally holes are not longer than 25 meters, as anything longer will likely have deviation issues. For sublevel stoping drilling can be done long in advance of the blasting.

There are three different drill patterns that are usually used with sublevel stoping. Fan drilling is where holes are drilled fanning out from the roof of the sublevel. The diagram to the right shows the pattern as it would be applied to sublevel mining. Fan drilling is mostly seen in sublevel caving, however can be applied to stoping in narrow stopes as well. These holes may be drilled upwards or downwards depending on the blasting convention chosen for the mine.

Another pattern commonly used is ring drilling, where the holes are drilled all the way around the working level so that they form a ring. The holes must all be on the same plane so that they can be blasted off as a unit. This pattern would typically be used in massive deposits as it allows for the holes to reach the sides of the deposit. The figure to the left shows how ring drilling would be applied to a typical sublevel operation.

The final drill pattern that can be used is parallel drilling; this application also works well in narrow mines. It involves drilling vertical holes in parallel lines. This drill pattern requires no special equipment and has many different applications in mining. For sublevel stoping in narrow orebodies this method is preferable as it does not drill into the walls of the stope. The figure to the right shows the parallel drill pattern.

The pattern that will be drilled depends on the drill chosen. For fan drilling there is a specific drill that does the job, called a mechanized fan drill. This drill has 2 booms that extend in a vertical direction. There are also drill rigs that work specifically for drilling rings in concentric circles. These rigs only drill in circles around vertical cross sections in the stope.
Before commencing production drilling it is important to consider many ore parameters. Important parameters include fragment size, drillability, and length of the hole, accuracy, orientation and spacing. Of late longhole drills have been the unit of choice for drilling.

Primary drilling is important because if done correctly it will lower the chances that secondary drilling will have to be done. Secondary drilling is done when the pieces of rock are too large for the drawpoints or equipment to handle. Large boulders must be drilled, loaded and blasted a second time in order to be removed. This is a time consuming and expensive task.
[5][2]

Blasting

Explosives

Choosing an explosive involves making sure you have a proper powder factor for the rock that will be blasted. The powder factor is defined as the quantity of explosive used per unit of rock blasted. This affects what the hole size, burden and spacing that will be used. These parameters can be preselected based on knowledge of the desired production rate of the stope.

The type of explosive chosen for a project usually depends on the economics of the project more than the mining method. ANFO (ammonium nitrate and fuel oil) is often used for sublevel stoping. ANFO can be free poured into a hole or pumped down pneumatically. Depending on the flow and content of water in the rock, a water gel or emulsion maybe used instead of ANFO.

Blast Sequencing

Blast sequencing will differ for every mine however there are a few general rules of thumb to keep in mind. Each ring should be set off as a unit. This removes a panel of ore from the working end of the drift allowing for retreat away from broken ore. The second blast will use the open space from the first blast to expand into. Usually the bottom sublevel will be blasted first and kept a few rounds ahead of the one above it. This is the same on the level above all the way to the top working level. The holes to be blasted together will be on the same plane regardless of the inclination of the holes. Some mines work from the top retreating farther back at the top and moving down to ensure that workers are not exposed to conditions such as unsupported ground above. Holes can be pre drilled for efficiency and can allow for multiple blasts at one time. This will allow for the bigger and more productive blasts and also improves the efficiency.
[2][5]

Equipment

Sublevel open stoping is a highly mechanized mining method utilizing a wide range of equipment for drilling and mucking. Typically production drilling is carried out by high-efficiency column and arm longhole drills or DTH drill rigs. These systems use electric drive instead of hydraulic and have high pressure pneumatic DTH hammers or rotary percussion drilling systems. It is with recent advantages in drilling technology that these systems have revolutionized sublevel stoping operations.
[5]

Mucking may be done with load haul dump equipment or LHD’s. They either take the ore from the draw point entrances and travel to an ore skip location for shaft transportation of ore to surface, or they transport and load the ore onto haul trucks which then transport the ore to surface via ramp access. Slusher and scraper systems have also been used in combination with mine car transport as well. In this scenario, the ore is scraped into designated ore passes where it falls into waiting mine cars. These cars then either transport the ore to waiting haul trucks or to skip locations as described previously. This system is utilized if there is good ore fragmentation, while LHDs handle larger pieces. If necessary, underground crushers can also be installed to crush ore to a more manageable size before transportation.
[11]

Support

Pillars

A wide array of pillars are used in a typical sublevel stoping operation. Rib pillars are installed as a support divider between horizontally adjacent stopes. During production, as vertical slices are blasted away, some slices are left behind as support pillars in order to help control subsidence within the stope. Sill and crown pillars are also a key support development in sublevel stoping. They are used between vertically adjacent stopes (sill pillar) or between a stope and surface operations (crown pillar). These pillars provide roof and floor support as well as aid in the prevention of cave-ins, rock bursting, and surface subsidence.
[4][2]

Backfill

In sublevel stoping a backfill program is usually established in large openings created during production. Backfill allows for the recovery of support pillars, permitting up to 90% ore recovery. Apart from this, backfill provides additional ground support, reduces dilution and helps with the redistribution of stresses around openings, which reduces rock bursting events.

Typically the types of backfill used are uncemented rock and sand fill, and cemented hydraulic/high density tailings fill made from the waste rock and tails produced from mining operations. Cemented tailings have also been known to warrant the elimination of rib pillars between stopes, which allows for a further increase in ore recovery. However, preparation and installation of cemented fill is costly and is not always economically justified, in this case recovery of the support and rib pillars is not practical.
[2]

Ground Support

Due to the strong nature of the rock required for use of this method, ground support is usually minimal. However ground conditioning is still needed in some areas and is done using different types of bolts including cable/rock bolts and grouted rebar. In Canada, any opening in which workers will be present needs to have a minimum bolt pattern before the workers are allowed to work freely.

Cable bolts of up to 150ft in length have proven extremely effective for full stope wall support. Installation is usually carried out during blasting development from within the production sublevels. Typical types of cable used for these bolts include hoisting cable and low –relaxation strand cable. Grouted rebar has also been proven successful for wall support using mainly resin or cement grout to anchor each bar into place.
[2]

Due to the large nature of the stope areas, it has been found that high horizontal stresses develop and cause deterioration of the development openings. Sufficient support is needed to combat this and in addition to bolting, mesh and shotcrete can be used in these areas of really large stress concentrations.
[5]

Safety

Overall sublevel stoping is a very safe method. Miners are not required to work on top of any broken ore or unsupported ground and the advancement process of the method requires retreating from mined out areas. The areas in which workers will be present, such as development drifts, are supported by rock bolts, cable bolts or other artificial supports.
[4]

Safety is further increased by use of mechanized equipment. Advances in wireless control technology allow for remote operation of haul equipment used in unsupported areas and for drills to be operated remotely from the collar of the hole. In order to maintain productivity, large volume blasts must be conducted. Worker safety is ensured by doing this from refuge stations or from surface, with no mine personnel in the blast area.

Ventilation in sublevel stoping is also very efficient. This is due to the large nature of stope areas and multiple stope access points. This allows for sufficient space for air to flow through and allows for safe and comfortable working conditions for all workers present in the stope and surrounding work area.
[2]

Economics

Cost Distribution

A breakdown of mining costs for a typical sublevel open stoping operation.[5]

The pie chart accurately represents the development intensive nature of sublevel stoping, with development activities accounting for one third of total mining costs. It should also be noted that labour costs account for roughly 40-50% of total mining costs and can be accounted for in a number of the sections above; mainly supervision and service, loading and hauling, and development.
[5]

Cost Minimization

In general sublevel stoping is a high production, yet low-cost method. It is a very popular method chosen when open pit mining activities are no longer economical and mines move to underground operations. The key to cost minimization when using this method is mechanization. Selection of the biggest equipment that the mine will permit allows for reduced total development in terms of drilling, and increased efficiency and production when loading and hauling.
[2]

Refer to the project economics section for a more in depth look into the economics of a typical mining operation.

This article was designed and created by Jonathan Scalise, Lauren Avery, Andrew Mountford, Danielle Piercey, and Dylan Labrech as part of the MINE 448 Underground Mine Design Course taught by Stephen McKinnon at Queen's University.